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author | Franciszek Malinka <franciszek.malinka@gmail.com> | 2022-08-24 23:22:12 +0200 |
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committer | Franciszek Malinka <franciszek.malinka@gmail.com> | 2022-08-24 23:22:12 +0200 |
commit | 37bb95dea0ac896d9c409fe64f3c1ce99ca43bb2 (patch) | |
tree | a6ff9e51b50fe9d6c47edddd4539846833788f76 /sections/examples.tex | |
parent | a15a0040023eb4f8f2b9d9653789063b86ccbe62 (diff) |
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-rw-r--r-- | sections/examples.tex | 63 |
1 files changed, 35 insertions, 28 deletions
diff --git a/sections/examples.tex b/sections/examples.tex index fdb8c6a..3d276ff 100644 --- a/sections/examples.tex +++ b/sections/examples.tex @@ -1,6 +1,41 @@ \documentclass[../lic_malinka.tex]{subfiles} \begin{document} + In this section we give examples and anti-examples of Fraïssé classes + with WHP or CAP. + + + \begin{example} + The class of all finite graphs $\cG$ is a Fraïssé class with WHP and free + amalgamation. + \end{example} + + We have already shown this fact. Thus get that the random graph has a generic + automorphism. + + \begin{example} + A $K_n$-free graph is a graph with no $n$-clique as its subgraph. + Let $\cG_n$ be the class of finite \emph{$K_n$-free} graphs. $\cG_n$ + is a Fraïssé class with WHP and free amalgamation. + \end{example} + + Showing that $\cG_n$ is indeed a Fraïssé class is almost the same as in + normal graphs, together with free amalgamation. WHP is trickier and the proof + can be seen in \cite{eppa_presentation} Theorem 3.6. Hence, $\Flim(\cG_n)$ + has a generic automorphism. + + \begin{example} + The class $\cV$ of all finitely generated vector spaces over a countable field + is a Fraïssé class with WHP and CAP. + \end{example} + + Vector spaces of the same dimension are isomorphic, thus it is obvious that + $\cV$ is essentially countable. Also $HP$ and $JEP$ are obvious, as we can + always embed space with smaller dimension into the bigger one. Amalgamation + works exactly the same. In fact, such amalgamation is indeed canonical. + + Now we give some anti-examples: + \begin{example} Let $\cL$ be the class of all finite linear orderings. Then: \begin{enumerate} @@ -155,32 +190,4 @@ Thus there cannot be a dense conjugacy class in $\Aut(\Sigma)$ and so there's no generic automorphism. - \begin{example} - The class $\cV$ of all finitely generated vector spaces over a countable field - is a Fraïssé class with WHP and CAP. - \end{example} - - Vector spaces of the same dimension are isomorphic, thus it is obvious that - $\cV$ is essentially countable. Also $HP$ and $JEP$ are obvious, as we can - always embed space with smaller dimension into the bigger one. Amalgamation - works exactly the same. In fact, such amalgamation is indeed canonical. - - \begin{example} - The class of all finite graphs $\cG$ is a Fraïssé class with WHP and free - amalgamation. - \end{example} - - We have already shown this fact. Thus get that the random graph has a generic - automorphism. - - \begin{example} - A $K_n$-free graph is a graph with no $n$-clique as its subgraph. - Let $\cG_n$ be the class of finite \emph{$K_n$-free} graphs. $\cG_n$ - is a Fraïssé class with WHP and free amalgamation. - \end{example} - - Showing that $\cG_n$ is indeed a Fraïssé class is almost the same as in - normal graphs, together with free amalgamation. WHP is trickier and the proof - can be seen in \cite{eppa_presentation} Theorem 3.6. Hence, $\Flim(\cG_n)$ - has a generic automorphism. \end{document} |